In the semiconductor industry, there is a continuing trend toward producing semiconductor wafers having higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down the device dimensions in the wafers. In order to accomplish such high device packing density, smaller and smaller features sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as comers and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and an exposing source of radiation (such as ultraviolet light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the mask, having a particular pattern. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the pattern formed in the mask. Once the image is projected, it is indelibly formed in the coating. Exposure of the resist to the radiation through the photomask causes the image area on the resist to become either more or less soluble (depending on the coating) in a particular developer solvent.
In order to form the actual patterns in the resist following exposure to radiation, the resist undergoes a subsequent development step in which a developer is applied thereto. The developer serves to remove the more soluble areas of the resist and leave behind the patterned image which will serve as a mask for etching of the underlying semiconductor layers of the wafer. Unfortunately, however, areas which are less soluble to the developer are also slowly eroded by the developer thereby distorting the desired resist pattern. Thus, it is beneficial to utilize a development process which exposes the resist to a developer for a minimum amount of time and in which the amount of developer applied to the resist at any given location is uniform so that resist erosion occurs evenly across the wafer.
Generally, there are three main methods for developing the photoresist: immersion developing, spray developing, and puddle developing. In immersion developing, several wafers are batched-immersed and agitated in a bath of developer. This development process has an advantage in that it allows for high throughput. However the exposure time of the resist to the developer is often long and the overall process typically does not allow for the tight critical dimension control required in the newer more densely populated semiconductor wafers.
In spray development, the developer is sprayed onto a resist at a preset optimal spray velocity while the wafer is spun at a high speed of approximately 1000 to 2000 rpm. Each wafer is treated with a fresh dose of developer solution. While spray development is generally an effective method to dissolve resist, it is often difficult to control the precise application of the developer to the wafer through the spray nozzle. For instance, during application of the developer a portion of the developer may be sprayed outside of the wafer surface. Thus, as a certain amount of developer is typically wasted, it takes a longer amount of time to apply the desired amount of developer to the resist at the preset spray velocity. As a result, the resist is exposed to the developer for a longer period of time than is often acceptable for the tight critical dimension control needed with respect to the denser semiconductor wafers being produced today.
In puddle development, a predetermined amount of developer is initially dispensed onto the resist surface while the wafer rotates at a relatively slow speed of approximately 50 rpm, for instance. As the developer is applied to the wafer, the spinning causes the developer to become uniformly spread over the surface. Application of the developer is typically accomplished using a generally rectangular shaped nozzle positioned over the wafer in a symmetrical, centered fashion so as to coincide with the axis of rotation of the wafer. As the wafer is rotated, the nozzle dispenses the developer onto the resist. Unfortunately, during application of the developer, a center portion of the wafer is continuously exposed to fresh developer while the remaining portions are only exposed to fresh developer at those instances where such portions pass under the rectangular shaped nozzle. This, in turn, causes a non-uniform layer of developer to be applied to the wafer thereby deleteriously impacting critical dimension control of the patterned resist.
Accordingly, there exists a need in the art for a method and apparatus of developing a resist which overcomes the drawbacks described above and others.